In 1982, a series of novel silicoaluminophosphate SAPO molecular sieves were successfully synthesized by the Union Carbide Corporation, which was disclosed in U.S. Pat. No. 4,310,440. Since then silicoaluminophosphate molecular sieve and its heteroatom-substituted derivatives have been one research focus in the field of materials and catalysis. Among these molecular sieves, silicoaluminophosphate SAPO-34 molecular sieve with CHA type framework has shown an excellent catalytic performance in methanol to olefins (MTO) process, due to its proper acidity and pore structure (Applied Catalysis, 1988, 40: 316).
SAPO-34 is a molecular sieve with chabazite-type framework containing 8-member ring ellipsoidal cage and 3-dimensional channel, which is formed by stacking of double six-rings according to ABC sequence. SAPO-34 is microporous molecular sieve with a pore size of 0.38×0.38 nm and cage size of 1.0×0.67 nm. Space group of SAPO-34 is R3m belonging to trigonal crystal system (J. Phys. Chem., 1990, 94: 2730). SAPO-34 is formed by Si, Al, P and O whose composition change at some range, generally in the order of n(Si)<n(P)<n(Al).
SAPO-34 molecular sieve is generally produced by a hydrothermal synthesis process which uses water as the solvent and is conducted in a sealed autoclave. The components for the synthesis comprise an aluminum source, a silicon source, a phosphorus source, a structural-directing agent and deionized water. The silicon source may be chosen from silica sol, active silica and orthosilicate ester. The aluminum source may be chosen from active alumina, pseudoboehmite and alkoxy aluminum. Preferable silicon source and aluminum source are silica sol and pseudoboehmite. Phosphorus source is generally 85% phosphoric acid. The structural-directing agent partly affects the microstructure, elemental composition, morphology of synthesized molecular sieve, thus producing an impact on the catalytic performance of synthesized molecular sieve. Preparation methods of multiple SAPO molecular sieves have been reported in U.S. Pat. Nos. 4,310,440 and 4,440,871, and the template agents used to synthesize SAPO-34 were tetraethyl ammonium hydroxide, isopropylamine, and a mixture of tetraethyl ammonium hydroxide and dipropylamine. A method for preparing SAPO-34 molecular sieve was published in Chinese patent ZL93112230 using triethylamine with low price as the temple agent, reducing the cost of synthesis. Hereafter, methods for preparing SAPO-34 molecular sieve were published in Chinese patent ZL93112015 and ZL94110059 using diethylamine and a mixture of diethylamine and triethylamine respectively, further reducing the cost of synthesis.
A method for preparing SAPO-34 molecular sieve was published in Chinese patent CN1131845C using multiple temple agents containing diisopropylamine. A method for preparing SAPO molecular sieves was published in international patent WO 03/040037A1 via a dry process of solid precursor, in which it was mentioned that diisopropylamine could be used as a template agent, and the product was uncertainly describe as SAPO molecular sieves, including SAPO-34 molecular sieve. It is worth noting that although the template agents included diisopropylamine, diisopropylamine was used as a template agent in none of above patent examples.
Usually, with the increase of Si content in SAPO molecular sieves, the Si coordination structures change from Si(4Al) to Si(nAl) (n=0 to 4) (in different kind of SAPO molecular sieves, the allowable maximum of single Si distribution in the frameworks are different, seeing J. Phys. Chem., 1994, 98, 9614). The Si coordination structures have significant effect on the acid density and the acid intensity, and the acid intensity is enhanced in the order of Si(1Al)>Si(2Al)>Si(3Al)>Si(4Al). In the other hand, the amount of acid center produced by each Si atom decrease with the appearance of Si islands in the framework of SAPO molecular sieves (Si(4Al) is 1, and the others are less 1), leading to the decrease of the acid density. It is supposed that using the SAPO molecular sieves as the acid catalyst, the catalytic performance must be effected by the distribution of Si in the framework since the non-uniform distribution of Si in crystal bring the non-uniform distribution of acidity. The enrichment of Si on the surface of crystal indicates that the Si coordination structures on the surface of crystal are more complex than inside the crystal. Weckhuysen et al have reported that in the process of methanol to olefin (MTO), reaction firstly occurs near the surface of crystal, and with the reaction going on, the large coke species form and block the pores progressively, making the diffusion of the products inside the crystal more difficult (Chemistry—A European Journal, 2008, 14, 11320-11327; J. Catal., 2009, 264, 77-87). It indicates that the acid environment on the surface of crystal is very important to the catalytic performance, and it is significant to seek a control method of the degree of Si enrichment on the molecular sieve surfaces.
Disclosure
An object of the present invention is to provide a SAPO-34 molecular sieve containing template agent diisopropylamine. The chemical composition in the anhydrous state of said molecular sieve is expressed as: mDIPA.(SixAlyPz)O2; wherein, DIPA is diisopropylamine existing in cages and pore channels of said molecular sieve; m is the molar number of diisopropylamine per one mole of (SixAlyPz)O2, and m is from 0.03 to 0.25; x, y, z respectively represents the molar number of Si, Al, P, and x is from 0.01 to 0.30, and y is from 0.40 to 0.60, and z is from 0.25 to 0.49, and x+y+z=1. There is a slight Si enrichment phenomenon on the crystal surface of said molecular sieve crystal, and the ratio of the surface Si content to the bulk Si content of the crystal ranges from 1.48 to 1.01; wherein the Si content is calculated by the molar ratio of Si/(Si+Al+P).
In X-ray diffraction spectrogram of said SAPO-34 molecular sieve, the diffraction peaks are included shown in Table 2. There is a slight Si enrichment phenomenon on the crystal surface of said molecular sieve crystal, and the ratio of the surface Si content to the bulk Si content of the crystal ranges from 1.48 to 1.01, preferably ranges from 1.42 to 1.02, further preferably ranges from 1.36 to 1.03, and more further preferably ranges from 1.33 to 1.03; wherein the Si content is calculated by the molar ratio of Si/(Si+Al+P). The Si contents from core to shell of said molecular sieve crystals increase uniformly or non-uniformly.
Another object of the present invention is to provide a method for preparing SAPO-34 molecular sieve.
Another object of the present invention is to provide a SAPO-34 molecular sieve prepared using the above method and catalysts prepared from the same for acid-catalyzed reaction or an oxygenate to olefins reaction.
The technical problem to be solved in the present invention is that the SAPO-34 molecular sieve with high purity is hydrothermally prepared directly using diisopropylamine as the template agent, and selecting the silicon source, the aluminum source, and the phosphorus source from traditional ingredients. There is a slight Si enrichment phenomenon on the crystal surface of said molecular sieve crystal, and the ratio of the surface Si content to the bulk Si content of the crystal ranges from 1.48 to 1.01; wherein the Si content is calculated by the molar ratio of Si/(Si+Al+P). Through experimental research, the inventers of the present invention found that the degree of Si enrichment on the molecular sieve surfaces can be decreased by adding a surfactant.
The present invention provides a hydrothermal method for preparing said SAPO-34 molecular sieve.
The present invention reports said method for preparing SAPO-34 molecular sieve, charactered in including the steps as follows:
(a) a silicon source, an aluminum source, a phosphorus source, a surfactant BM, deionized water and structural-directing agent DIPA are mixed, and an initial gel mixture with following molar ratio is obtained:    SiO2/Al2O3 is from 0.05 to 1.5;    P2O5/Al2O3 is from 0.5 to 1.5;    H2O/Al2O3 is from 16 to 150;    DIPA/Al2O3 is from 2.0 to 5.9;    BM/Al2O3 is from 0.001 to 0.05;
(b) the initial gel mixture obtained in said step (a) is transferred into a synthetic kettle, then sealed and heated to crystallization temperature range from 150° C. to 220° C., crystallized for crystallization time range from 0.5 h to 72 h under an autogenous pressure;
(c) after finishing the crystallization, the solid product is centrifugal separated, washed to neutral using deionized water and dried to obtain said SAPO-34 molecular sieve;
wherein, said structural-directing agent DIPA is diisopropylamine; said surfactant BM is alkyl ammonium halide.
In said step (a), the silicon source is one or more selected from silica sol, active silica, orthosilicate esters and metakaolin; the aluminum source is one or more selected from aluminum salts, activated alumina, aluminum alkoxide and metakaolin; the phosphorus source is one or more selected from phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, organophosphorous compounds and phosphorus oxides.
Said surfactant BM is alkyl ammonium halide. Preferably said surfactant BM is one or more selected from dodecyl trimethyl ammonium chloride, tetradecyl trimethyl ammonium chloride, hexadecyl trimethyl ammonium chloride, octadecyl trimethyl ammonium chloride, dodecyl trimethyl ammonium bromide, tetradecyl trimethyl ammonium bromide, hexadecyl trimethyl ammonium bromide, octadecyl trimethyl ammonium bromide.
In the initial gel mixture obtained in said step (a), the preferable molar ratio of H2O/Al2O3 is from 26 to 120, and further preferably the molar ratio of H2O/Al2O3 is from 31 to 100.
In the initial gel mixture obtained in said step (a), the molar ratio of DIPA/Al2O3 is from 3.0 to 5.0.
In the initial gel mixture obtained in said step (a), the molar ratio of BM/Al2O3 is from 0.001 to 0.03.
In said step (b), preferably the crystallization condition are the crystallization temperature range from 170° C. to 210° C. and the crystallization time range from 1 h to 60 h; and further preferably the crystallization condition are the crystallization temperature range from 180° C. to 210° C. and the crystallization time range from 1 h to 24 h; and more further preferably the crystallization condition are the crystallization temperature range from 190° C. to 210° C. and the crystallization time range from 1 h to 12 h.
In said step (b), the crystallization is carried out dynamically or statically.
The SAPO-34 molecular sieves prepared by said methods can be used as catalysts for acid-catalyzed reaction after calcining at a temperature from 400 to 700° C. in air.
The SAPO-34 molecular sieves prepared by said methods can be used as catalysts for an oxygenate to olefins reaction after calcining at a temperature from 400 to 700° C. in air.
The present invention also refers to a catalyst for acid-catalyzed reaction, which is obtained by calcining at least one of said SAPO-34 molecular sieves or at least one of the SAPO-34 molecular sieves prepared by said methods, at a temperature from 400 to 700° C. in air.
The present invention also refers to a catalyst for an oxygenate to olefins reaction, which is obtained by calcining at least one of said SAPO-34 molecular sieves or at least one of the SAPO-34 molecular sieves prepared by said methods, at a temperature from 400 to 700° C. in air.
The present invention can bring the advantages including:
(1) obtaining a SAPO-34 molecular using diisopropylamine as the template agent, characterized in a slight Si enrichment phenomenon on the crystal and with the ratio of the surface Si content to the bulk Si content of the crystal ranging from 1.48 to 1.01.
(2) the SAPO-34 molecular sieves prepared by said methods in present invention having excellent catalytic performance in the MTO reaction and ethanol dehydration reaction.